On Mon, Jun 10, 2024 at 1:28 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote:
>
> On Sat, Jun 8, 2024 at 8:53 AM Takero Funaki <flintglass@xxxxxxxxx> wrote:
> >
> > This patch fixes zswap global shrinker that did not shrink zpool as
> > expected.
> >
> > The issue it addresses is that `shrink_worker()` did not distinguish
> > between unexpected errors and expected error codes that should be
> > skipped, such as when there is no stored page in a memcg. This led to
> > the shrinking process being aborted on the expected error codes.
> >
> > The shrinker should ignore these cases and skip to the next memcg.
> > However, skipping all memcgs presents another problem. To address this,
> > this patch tracks progress while walking the memcg tree and checks for
> > progress once the tree walk is completed.
> >
> > To handle the empty memcg case, the helper function `shrink_memcg()` is
> > modified to check if the memcg is empty and then return -ENOENT.
> >
> > Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
> > Signed-off-by: Takero Funaki <flintglass@xxxxxxxxx>
> > ---
> > mm/zswap.c | 31 ++++++++++++++++++++++++++-----
> > 1 file changed, 26 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/zswap.c b/mm/zswap.c
> > index d720a42069b6..1a90f434f247 100644
> > --- a/mm/zswap.c
> > +++ b/mm/zswap.c
> > @@ -1393,7 +1393,7 @@ static struct shrinker *zswap_alloc_shrinker(void)
> >
> > static int shrink_memcg(struct mem_cgroup *memcg)
> > {
> > - int nid, shrunk = 0;
> > + int nid, shrunk = 0, stored = 0;
> >
> > if (!mem_cgroup_zswap_writeback_enabled(memcg))
> > return -EINVAL;
> > @@ -1408,9 +1408,16 @@ static int shrink_memcg(struct mem_cgroup *memcg)
> > for_each_node_state(nid, N_NORMAL_MEMORY) {
> > unsigned long nr_to_walk = 1;
> >
> > + if (!list_lru_count_one(&zswap_list_lru, nid, memcg))
> > + continue;
> > + ++stored;
> > shrunk += list_lru_walk_one(&zswap_list_lru, nid, memcg,
> > &shrink_memcg_cb, NULL, &nr_to_walk);
> > }
> > +
> > + if (!stored)
> > + return -ENOENT;
> > +
>
> Can't we just check nr_to_walk here and return -ENOENT if it remains as 1?
>
> Something like:
>
> if (nr_to_walk)
> return -ENOENT;
> if (!shrunk)
> return -EAGAIN;
> return 0;
>
> > return shrunk ? 0 : -EAGAIN;
> > }
> >
> > @@ -1418,12 +1425,18 @@ static void shrink_worker(struct work_struct *w)
> > {
> > struct mem_cgroup *memcg = NULL;
> > struct mem_cgroup *next_memcg;
> > - int ret, failures = 0;
> > + int ret, failures = 0, progress;
> > unsigned long thr;
> >
> > /* Reclaim down to the accept threshold */
> > thr = zswap_accept_thr_pages();
> >
> > + /*
> > + * We might start from the last memcg.
> > + * That is not a failure.
> > + */
> > + progress = 1;
> > +
> > /* global reclaim will select cgroup in a round-robin fashion.
> > *
> > * We save iteration cursor memcg into zswap_next_shrink,
> > @@ -1461,9 +1474,12 @@ static void shrink_worker(struct work_struct *w)
> > */
> > if (!memcg) {
> > spin_unlock(&zswap_shrink_lock);
> > - if (++failures == MAX_RECLAIM_RETRIES)
> > +
> > + /* tree walk completed but no progress */
> > + if (!progress && ++failures == MAX_RECLAIM_RETRIES)
> > break;
>
> It seems like we may keep iterating the entire hierarchy a lot of
> times as long as we are making any type of progress. This doesn't seem
> right.
>
Ah actually, reading your comment closer then yeah the
shrink-until-accept is the intended-ish behavior (from Domenico's
older patches to re-work zswap reclaim). The one-page-per-attempt is
also "intended" behavior, but not any of our's intentions - just
something that remains from the past implementation as we rework this
codebase one change at a time :)
